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Generating Electricity

Most methods of electricity generation use a generator (and its two key components): coils of wire and magnets

Moving a wire a certain way in a magnetic field creates a flow of electricity

Physics Curriculum Connections

Physics 12:

Electromagnetism

Physics 12:

Analyse electromagnetism, with reference to magnetic fields and their effects on moving charges

Applications of Physics 12:

Electricity and Magnetism

Applications of Physics 12:

Analyse the operation and applications of motors, alternators, and generators

How do we generate electricity? Today there are a lot of different answers to that question: Coal, Natural Gas, Nuclear, Wind, Solar and Tidal power are all viable options for electrical generation. However did you know that all of these methods except solar come down to the same thing? They extract energy in order to drive something called a generator that takes any rotating force (or torque) and turns it into electricity.

Parts of a Generator
There are two key elements in every generator:

These elements are always arranged so that the magnets can spin past the coils of wire, or vice versa.

So what is it about magnets and coils of wire that generates electricity? Let’s quickly review a few things about magnets.

Review of Magnets

The basic thing that magnets do is attract some types of metal and also attract or repel other magnets according to their polarity. We often use an idea called a magnetic field to describe the interactions of a magnet. The direction of the magnetic field lines shows the direction that the magnetic field would push another north magnetic pole, and the density of the magnetic field lines shows the strength of the magnetic field at that location (or how strong the push would be).

From looking at the diagram below we can see that the magnetic field is strongest at the poles (where the density of lines is highest) and that the north pole would push another north AWAY from itself. Also we can see that the strength of the magnetic field gets weaker when we are far away from the magnet. We already knew those things, but it is good to review.

So we know that generating electricity involves magnets and wires. To generate electricity can we just put a loop of wire next to a magnet?

Sadly, no. it turns out that we need to CHANGE something. If we hold a magnet still near a coil, we don’t generate any electricity. If we hold a magnet still FAR from a coil, we still don’t. It’s only when we move the magnet either towards or away from the coil that we generate electricity.

To figure out what’s going on here, we need to look more closely at what the magnet does to a moving wire.

Magnetic Fields and Wires
So I’ll just spit it out: when you move a wire through a magnetic field, the magnetic field pushes all the electrons to one side of the wire! Well, I should clarify: you have to orient and move the wire in a very special way.

Firstly, the wire needs to be moved perpendicular to the magnetic field. If you move it in the same direction as the magnetic field, nothing happens.

Secondly, in order to make useful electricity, we want to push the electrons down the LENGTH of the wire. In order to do this, you need to move the wire sideways. If you move the wire in the same direction as the length of the wire, the magnetic field pushes the electrons to the sides of the wire (which is not very useful).

In wire A, charges are pushed to either end of the wire, resulting in a useful voltage
In wire B, charges are pushed to the top and bottom of the wire, but this does not result in a useful voltage.
In wire C, no charges are pushed anywhere because the motion of the wire is parallel to the magnetic field.

The question of why this happens has many answers, each more mysterious than the last, but for now let’s leave it that this is a fact of nature and it Just Happens. (Teacher note: this might be a good time for a demo)

In summary, if your velocity is perpendicular to the magnetic field, there will be a force that pushes charges in a direction perpendicular to both the velocity AND the magnetic field. If the magnetic field is in the other direction, the push will be in the other direction. This is summarized by something called the “Right Hand Rule”, which is explained here.

So how do we make electricity with this effect?

That seems easy, right? We just hook up some wires to either end of this wire and run it through a magnetic field. Let’s also hook it up to a lightbulb, which represents something that USES the electricity.

Unfortunately this doesn’t work! We can see why by thinking about each of the wires individually: the two on the top and bottom have the charges pushed to the side of the wire, which doesn’t really create useful electricity so they don’t help us. The two on the side both have the positive charges pushed to the top, but that’s the problem: in order to have useful electricity we need a net flow of current AROUND the loop. Having a push upward on the lefthand wire AND a push upward on the righthand wire means these pushes are canceled out and we get no flow of current.

Each of the vertical wires has its positive charges pushed to the top of the wire. These pushes cancel out, so there is no net flow of charge and that means no electricity!

So we can see that moving a loop through a constant field won’t work. What can we try that WILL work?

Question: Which of the following will NOT light up the bulb?

Answer: C and D

Question: Which of these will light the bulb most strongly?

Answer: B

We get the best results when one side of the loop is in a field Upwards, and the other side is in a field Downwards. This way the pushes on the charges push in the same direction AROUND the loop.

Application in the Real World

Situation B above gives us a very good picture of how modern generators work! They use magnets of alternating polarity to set up alternating fields and then move coils of wire past them.

This diagram shows a generator that varies the B through each coil quite rapidly. The two parts are shown separated, but when assembled the ring of coils will be sitting on the axle of the ring of magnets below. Each magnet has the opposite polarity of its neighbors, so as an individual coil rotates around it will see a North pole followed by a South pole followed by a North etc.

Image sourced from The Norweigian University of Science and Technology 2

You can also achieve the same effect by moving the magnets instead of the coils: you still get a relative velocity between the magnets and coils, and on each transition between adjacent magnets you get a push around the coil to create electricity.

This is another type of generator. The coil is rotated in such a way that each side of the coil is moving in opposite directions in a constant magnetic field. This means that both pushes will always be in the same direction around the coil and you will generate electricity.

This setup is very easy to build, but it has one interesting feature: the voltage generated by a generator like this is not always in the same direction. The generated voltage changes direction twice each cycle. This type of electricity is called “Alternating Current”, and it has some very useful properties that we will investigate in future lectures. It is also the kind of electricity that you find in your walls!

For some extra reading, a good animation of Electric Motors can be found here.